Alloy steel pipes used in power plants, chemical and petro-chemical plants and refineries require stringent and verifiable temperature control during pre- and post-welding. In the case, for example, of Cr—Mo—V P91, the area to be welded (hereafter the weld area) must be pre-heated to a specific temperature before welding is performed. As is well known, an untreated weld is subject to the risk of hydrogen cracking due to residual stresses. In order to eliminate this risk, post-weld heat-treatment methods are applied to the weld area. Post-weld heat-treatment is the process of heating a metal component to a sufficient temperature below its transformation temperature, holding the metal component at that temperature for a predetermined amount of time, followed by uniform cooling. Typical stress relief temperatures for steel pipes range from 600° F. to 1650° F.
Traditional heat-treatment methods for pipe welds utilize standard power supplies hard-wired to heating cables, which are wrapped around the weld area. Thermocouples are also attached to the weld area. A thermocouple is a device that measures temperature by converting heat energy into electrical energy. The thermocouple is spot-welded at one end to the weld area. At the other end the thermocouple wires, called leads, are connected to an on-off controller located in a heat-treatment unit. This provides the on-off control to the heater cables in order to achieve the desired temperatures-time cycle.
A conventional heat-treatment unit contains a local controller that can be programmed to provide the appropriate temperature to the weld area through the heating cables. It also contains a strip chart temperature recorder, which records and displays the data locally. A plurality of on-site technicians is required to carry out the various tasks involved in the process, such as installing the heater cables and monitoring the temperature charts of long heat-treatment cycles.
The conventional method of heat-treatment is expensive, inflexible and only provides real-time status and operational temperature checking to technicians at the site of the heat-treatment unit. It is preferable to provide constant and real-time temperature monitoring, as well as a reduction in the number of on-site technicians.